Bcl-xL induces Drp1-dependent synapse formation in cultured hippocampal neurons

Maturation of neuronal synapses is thought to involve mitochondria. Bcl-x_L protein inhibits mitochondria-mediated apoptosis but may have other functions in healthy adult neurons in which Bcl-x_L is abundant. Here, we report that overexpression of Bcl-x_L postsynaptically increases frequency and amplitude of spontaneous miniature synaptic currents in rat hippocampal neurons in culture. Bcl-x_L, overexpressed either pre or postsynaptically, increases synapse number, the number and size of synaptic vesicle clusters, and mitochondrial localization to vesicle clusters and synapses, likely accounting for the changes in miniature synaptic currents. Conversely, knockdown of Bcl-x_L or inhibiting it with ABT-737 decreases these morphological parameters. The mitochondrial fission protein, dynamin-related protein 1 (Drp1), is a GTPase known to localize to synapses and affect synaptic function and structure. The effects of Bcl-x_L appear mediated through Drp1 because overexpression of Drp1 increases synaptic markers, and overexpression of the dominant-negative dnDrp1-K38A decreases them. Furthermore, Bcl-x_L coimmunoprecipitates with Drp1 in tissue lysates, and in a recombinant system, Bcl-x_L protein stimulates GTPase activity of Drp1. These findings suggest that Bcl-x_L positively regulates Drp1 to alter mitochondrial function in a manner that stimulates synapse formation.